Somali pirates who seized a Saudi supertanker with two British crew on board claim to have released the vessel and its crew today after receiving a $3m (£1.9m) ransom.

The leader of the group, which hijacked the Sirius Star in November, told the AFP news agency that all the pirates had left the vessel, which was moving into safe waters. "The ship is free, the crew is free," Mohamed Said said by phone from the pirate lair of Harardhere.

An associate of the pirates said a ransom had been paid to secure the release. "Pirates holding the Saudi ship took $3m yesterday evening and then released the ship this morning," Farah Osman told Reuters from Haradhere port.

He said the pirates had wanted more money before agreeing on the final sum. The barrels on board – worth $100m – represent more than a quarter of Saudi Arabia's daily crude oil output.

"The pirates are now arguing about division of the money," Osman said.

A regional maritime group said the Sirius Star, which was carrying 2m barrels of crude oil when it was hijacked, was moving south-east.

Andrew Mwangura, a spokesman for the East African Seafarers Assistance Programme, said: "The last batch of gunmen have disembarked from the Sirius Star. She is now steaming out to safe waters. But often when a deal has been released, the hijackers accompany the ship away from the coast like this."

The Sirius Star, a 300-metre Saudi-owned vessel, was hijacked by the pirates in November along with 25 crew members, including two Britons, in unpatrolled waters 450 miles south-east of the Kenyan port of Mombasa. The ship was then anchored off the coast of Somalia, where it has sat until now. The other crew members were from Poland, Croatia, the Philippines and Saudi Arabia.

A dramatic increase in piracy off the Somalian coast, in the Indian Ocean and in the Gulf of Aden – one of the world's most important shipping lanes – has led to increased patrols by international warships, including Nato forces and the first European joint naval operation. Last month China sent two destroyers to back international efforts to combat piracy in the area, the first operation of its kind.

Shipping insurance prices have also been sent soaring and some companies have chosen to navigate a much longer route around South Africa instead of going through the Suez canal.

Other vessels still being held by pirates include the MV Faina, a Ukrainian ship carrying 33 battle tanks.

Ransoms are often paid to pirates after lengthy negotiations and in 2008 Somali pirates, who often attack ships using small launches and are heavily armed, were believed to have netted more than £20m.

Vela International, the Dubai-based shipping arm of Saudi Aramco, which operates the ship, was not available for comment today but Salah Kaaki, the company's president, has said its "first and foremost priority" was ensuring the safety of its crew.